1. Field of the Invention
The present invention concerns a method for producing calcium carbonate from alkaline industrial waste or by-products, as well as a method for extracting calcium carbonate and vanadium from the mentioned waste or by-products.
2. Description of Related Art
Iron and steel manufacturing is one of the biggest industries in the world, producing annually more than one billion metric tons of steel. Large amounts of slag are produced as a by-product from iron- and steelmaking processes (annually 300-400 million metric tons worldwide). Current uses for steelmaking slag are cement aggregate, road construction, fertilizers, and liming material. The steel industry also accounts for approximately 6-7% of the total anthropogenic CO2 emissions to the atmosphere. The slag does, however, contain many components, such as Ca, Si, Fe, Mg, Al, Mn, V, and Cr, which could be valuable when separated from the rest of the slag.
Synthetic calcium carbonate, or precipitated calcium carbonate (PCC), is today produced by three different processes: a lime-soda process, a calcium chloride process, and a calcination/carbonation process. In the lime-soda process, calcium hydroxide is reacted with sodium carbonate to produce a sodium hydroxide solution, from which the calcium carbonate is precipitated. In the calcium chloride process, calcium hydroxide is reacted with ammonium chloride, forming ammonia gas and a calcium chloride solution. After purification, this solution is reacted with sodium carbonate to form a calcium carbonate precipitate and a sodium chloride solution. In the third, and most commonly used, production process, calcium oxide is hydrated with water, producing a calcium hydroxide slurry. The slurry is reacted with a CO2-rich flue gas, from which the calcium carbonate is precipitated.
The common PCC production processes require calcium oxide or -hydroxide as raw material, which is typically produced by calcining (i.e. burning) limestone, and causes significant CO2 emissions. The virgin limestone used also needs to have low levels of impurities in order to affect the quality of the PCC.
The prior art contains some examples of processes for separating various components from alkaline industrial waste or by-products, such as iron- and steelmaking slag. An example is the separation of alkali metals from the slag by converting them into carbonates using carbon dioxide (CO2) gas in water. This procedure is described, for example, in U.S. Pat. No. 4,225,565, EP 0263559, U.S. Pat. No. 5,466,275 and JP 57111215.
Further, JP 2005097072 provides a method for manufacturing an alkaline earth metal carbonate, wherein a gas containing carbon dioxide is contacted with an aqueous solution containing an alkaline earth metal and a salt of a weak base and a strong acid.
KR 20040026382 provides a method for manufacturing calcium carbonate using desulphurized slag and carbon dioxide or a CO2-containing exhaust gas. In this method, the desulphurized slag is added to water, the pH is adjusted to ≧12 and the obtained calcium-eluted solution is reacted with CO2 or a CO2-containing exhaust gas under a pH of ≧7.
JP 2007022817 provides a method of treating a steelmaking slag, wherein CaO present in the steelmaking slag is carbonated using a CO2-gas.